1. Field of the Invention
The invention relates to an adhesive varnish, an adhesive film and a wiring film. In particularly, the invention relates to an adhesive varnish that, when being used as an adhesive etc., can provide excellent storage stability, adhesive properties and heat resistance and moisture-resistance reliability after the adhesion. Also, the invention relates to an adhesive varnish that, when being used as an adhesive film or a wiring film, can prevent the foam formation and curling during the drying.
2. Description of the Related Art
In recent years, electric/electronic devices have become smaller to save space and lighter and internal wiring materials used therein are required to be fine and thin wirings to realize high density wiring. In addition, in order to reduce use of substances of concern, it has been progressively switched from conventional lead solder to lead-free solder. Accordingly, internal wiring materials are required to have improved heat resistance.
An insulation layer of the wiring member mentioned above is basically composed of a base film and an adhesive film (see, e.g., JP-A-H05-029399). Organic insulating films using heat-resistant films of polyimide, polyetherimide, polyphenylene sulfide or polyether ether ketone, etc., and heat-resistant composite films of epoxy resin-glass cloth or epoxy resin-polyimide-glass cloth, etc., are described as examples of the base film. For the adhesive layer, an adhesive containing a polyamide resin and an epoxy resin is disclosed.
However, the adhesive described in JP-A-H05-029399 has a problem that storage stability is poor due to high reactivity of an amino group present in a polyamide resin structure with the epoxy resin. In order to solve this problem, an adhesive composed of a phenoxy resin having epoxy groups at both terminals, acrylic rubber and a curing agent has been proposed (see, e.g., JP-A-2004-136631). The phenoxy resins of bisphenol A-type, bisphenol F-type, bisphenol AD-type, bisphenol S-type and copolymerization type having bisphenol A-type and bisphenol F-type are described as examples. The adhesive described in JP-A-2004-136631 has problems that adhesive strength is only about 0.5 kN/m even though the phenoxy resin considered to be relatively excellent in adhesive strength is blended and also solder heat resistance is as slightly low as 260° C.
An adhesive containing a thermoplastic polyurethane resin having a weight average molecular weight of 80,000 to 800,000, an epoxy resin and an epoxy resin curing agent has been disclosed as a measure of solving the above-mentioned problems (see, e.g., JP-A-2010-150437).
Although typical polyurethane resins have a problem in storage stability of the adhesive film because of high reactivity with epoxy resin, JP-A-2010-150437 describes use of a polyurethane resin having a molecular weight within a specific range to improve storage stability. Adhesive strength is 1.1 to 1.7 kN/m.
Meanwhile, it has been disclosed that an adhesive containing a polyurethane resin, an epoxy resin and a novolac resin having a specific structure has solder heat resistance of 300° C. (see, e.g., JP-A-2010-143988). However, it is known that the polyurethane resins used in JP-A-2010-150437 and JP-A-2010-143988 are generally depolymerized at a temperature of not less than 200° C. Heat resistance of polyurethane is generally 80 to 100° C. and there is therefore concern about use of adhesives containing a polyurethane resin in the industries and the field of automobile electronic devices which require high heat resistance.
An adhesive containing a (meth)acrylic-modified phenoxy resin and an epoxy resin and an adhesive containing a (meth)acrylic-modified phenoxy resin in the same manner, an urethane acrylate oligomer and a silane coupling agent have been also disclosed (see JP-A-2001-262111 and JP-A-2008-258607). The adhesive containing an epoxy resin described in JP-A-2001-262111 is excellent in heat resistance but has a problem that adhesive strength is as low as about 0.6 kN/m. Meanwhile, the adhesive containing a urethane acrylate and a silane coupling agent described in JP-A-2008-258607 is excellent in adhesion with metal conductors but there is lack of consideration of adhesion with polyimide films which are high heat-resistant base films.
Furthermore, for handling the adhesive film, it is important that curling is as small as possible and no tuck is present.